If you want to set the sights up head down to the Greyhound shooting club.
It costs £5 to shoot on the target range all day. And you can take AEG's. They don't sell ammo as it's an air rifle club.
But it will give you a seated shooting experience with your gun, and put some BB's on target. Set the sights ect.
Don't expect to be hitting the air rifle targets! Airsoft isn't that accurate, but the logs and white boards that are set out are good indicators, and the range is set up with yard markers. If you can make a BB fly from one corner to the other then that is about the max range an AEG can have.
We go down every now and again when we have guns to sort out. You can spend the day plinking and no one will bother you. There are a few rules the main one been the whistle. If it blows it's guns down till it blows again (someone is setting up a target and walking onto the range).
http://www.greyhoundgunclub.co.uk/about.html